1. Field of the Invention
This invention relates to printers and particularly to serial line printers.
2. Description of Prior Art
Serial line printers are well known in which a print mechanism is moved along a fixed line relative to a stationary print medium. The print mechanism can take various forms such as a type wheel, disk, sphere or cylinder (which print whole characters or other data symbols at various positions of a print line on a print medium), or one or more wires, electrodes, ink jets or other markers (which print characters, etc. using dots or other marks in groups or patterns). Printing may occur on-the-fly or when the print mechanism is at rest relative to the print medium and may be performed when the print mechanism traverse occurs in one or both directions. Regardless of the type of print mechanism used or the manner of printing, it is imperative that the lateral relative movement of the print mechanism be very precisely controlled in order to assure making the print impression at precise locations, if high quality printing with desired legibility is obtained.
It has been a long time practice to mount the print mechanism on a carrier which is movable on a horizontal bar or guide and to drive the carrier and print mechanism with a stepwise or continuous motion along the bar. A well-known drive mechanism comprised an electric motor connected by gearing, drive belts, feedscrews or the like to the print mechanism or carrier. See for example, U.S. Pat. Nos. 3,882,988, issued to Charles T. Sloan et al on May 13, 1975, and 3,985,463, issued to James J. Boyce et al on Sept. 16, 1975. The drive motor is controlled by circuits activated to control its direction and motion to cause the desired location movement of the print mechanism It is well known that such mechanisms are relatively complex and costly and contain certain inherent defects such as backlash or play. Such defects can cause positioning errors which affect the quality of printing. The magnitude of the error increases with use due to wearing and requires increasing service and adjustment to maintain desired accuracy.
It has been proposed in the prior art to eliminate the backlash and wear problems associated with the gear, belt, and feedscrew drive mechanisms by using a linear type electric motor which essentially comprises a bar of high-permeability magnetic material which passes through a solenoid attached directly to the print mechanism carrier. Examples of such arrangements are shown in U.S. Pat. Nos. 3,618,514 issued to A. Nyman et al on Nov. 9, 1971; 3,688,035 issued to G. Cless on Aug. 29, 1972 and 3,867,675 issued to N. Kitz et al on Feb. 18, 1975. In these patents, the linear motors are of the Lorenz force type having no natural detenting properties and the force properties are relatively low. Thus, print mechanisms transported by these motors must be relatively light weight and print mechanism positioning or motion with precision is not easily achieved.
It is a further practice in serial line printers to use a signal generator as part of the controls for timing the operation of the printer and/or motion of the print mechanism. Common signal generators comprise a grid extending along the print line or an encoder wheel attached to the drive motor. It always requires great care to locate the grid and encoder wheels relative to the print positions in the print line.